Method for producing tungsten carbides by gas-phase carburization

ABSTRACT

The invention relates to a method for producing wolfram carbides by gas-phase carburetion of wolfram powders and/or suitable wolfram precursor compounds in powder form at temperatures above 850° C. According to the method a CO 2 /CO mixture with a CO 2  content greater than the Boudouard equilibrium content corresponding to the carburetion temperature is used as carburetion gas phase.

The invention relates to a process for the direct preparation ofcarbides from tungsten-containing compounds by means of an atmospherecontaining carbon monoxide and carbon dioxide.

The reaction of tungsten-containing compounds, particularly tungstenoxides, with carbon monoxide, and optionally a mixture of carbonmonoxide and carbon dioxide at elevated temperature, is inherently wellknown.

U.S. Pat. No. 4,172,808 discloses a process in which WO₃ is converted totungsten carbide powders at temperatures from 590° C. to 680° C. in astream of carbon monoxide containing 5% to 10% carbon dioxide. Theproduct still contains 2% oxygen so it has not reacted completely.Moreover, the product has on its surface an unspecified amount of freecarbon. According to U.S. Pat. No. 4,172,808, the oxygen content isacceptable for catalyst applications and the superficial free carbon isrequired for a high catalyst activity. Tungsten carbide powders of thiskind are unsuitable, however, for use as a hard solid in hardmetalsbecause exact control of the carbon content to within a few hundredthsof a percent is important in this case.

According to U.S. Pat. No. 5,230,729, gas phase carburization by meansof a CO₂/CO mixture is described for a partial stage of the preparationof fine-particle WC—Co powder for hardmetal production. According tothis patent, the tungsten precursor compound Co(en)₃WO₄ is reducedinitially in a hydrogen-containing stream of inert gas to highly porousCo—W metal, then carburized in the carbon monoxide gas stream to WC—Coand free carbon is then removed in a CO₂/CO gas stream. Carburizationtemperatures from 700° C. to 850° C. are used in this case.

U.S. Pat. No. 5,230,729 also refers to a prior art according to whichcarburization to WC—Co powder with carbon activities from 0.35 to 0.95was evidently carried out without the intermediate step of hydrogenreduction to Co—W. The fact that, in view of the substantial carbonmonoxide decomposition brought about catalytically even at lowtemperatures due to the presence of cobalt, considerable absorption ofcarbon by the WC—Co starting substances takes place, leading to ametastable intermediate phase, is regarded as a disadvantage. As aresult, very long reaction times are required.

In view of the absence of a catalytic effect of cobalt, the teachingsregarding gas phase carburization for the preparation of WC—Co are nottransferable to the preparation of WC powders.

Extensive research was disclosed by LEMAITRE, VIDICK, DELMON in ActaChim. Acad. Sci., Hung. 111 (1982) pp. 449–463 and Journal of Catalysis99 (1986) pp. 415–427 for the preparation of tungsten carbide powders bygas phase carburization, wherein both carbon monoxide and mixtures ofcarbon dioxide and carbon monoxide with carbon dioxide contents from 9%to 50% were used in the temperature range from 772° C. to 850° C. Bothpowder with high proportions of free carbon and highly under-carburizedtungsten carbide powder or W₂C powder were obtained; in some cases,reoxidation to tungsten oxide was also observed. A carburizationtemperature of 750° C. is regarded as optimal.

A combined summary of the prior art seems to be that gas phasecarburization with pure carbon monoxide at temperatures above 850° C.leads to a coating of the precursor compound with graphite-like carbon,in view of the Boudouard equilibrium, with the result that the reactionis inhibited or brought to a standstill and hence long reactions whichare not feasible, at least on an industrial scale, are required. On theother hand, the tests of the prior art disclosed seem to prove that, ifCO₂/CO carburization gas mixtures are used with CO₂ contents which, atcarburization temperature, roughly correspond to the position of theBoudouard equilibrium or above, complete carburization is not possible.

It has now been found that substantially complete carburization oftungsten precursor compounds is obtained if, in the temperature rangefrom 800° C. to 1,000° C., preferably 850° C. to 950° C., thecarburization gas used has a carbon dioxide content, based on carbondioxide and carbon monoxide, which is above the Boudouard equilibrium atcarburization temperature, i.e. has a carbon activity of less than 1.If, however, the carbon dioxide content is too high, incompletecarburization will take place and even incomplete reduction. Accordingto the invention, the carbon activity of the CO/CO₂ mixture should bepreferably from 0.4 to 0.9, particularly preferably from 0.5 to 0.85.

The relationship between the relative proportions of CO and CO₂ in thecarburization gas on the one hand and the carbon activity a_(c) on theother hand is calculated from the following formula:1n a _(c)=1n(p ² _(co) /p _(co2))+20715/T−21.24,wherein p_(co) and p_(co2) denotes the partial pressure of CO and CO₂respectively in each case and T denotes the absolute temperature in K.The Boudouard equilibrium corresponds to a carbon activity a_(c)=1.

Due to the fact that the carbon activity during the process is keptbelow one, the separation of elemental carbon is renderedthermodynamically impossible, so the carbon content of the tungstencarbide obtained can be controlled precisely and in a reproduciblemanner. On the other hand, at temperatures above 800° C., preferably850° C., the CO₂ equilibrium concentration is already so low that evenif the equilibrium concentration is exceeded, complete and sufficientlyrapid reduction and carburization of the tungsten precursor compoundtakes place. Particularly preferably, the CO₂/CO partial pressure ratioshould not exceed 1:8.

The process according to the invention is surprising against thebackground of the phase diagram of the WO₃—WO₂—W—W₂C—WC—C systemdisclosed in the publication Journal of Catalysis 99, p. 430, FIG. 5,because, according to the phase diagram, the phase W₂C should form above800° C. due to carburization with a CO₂/CO mixture with a CO₂ contentabove the Boudouard equilibrium, and carburization to WC should not takeplace, at any rate within industrially feasible reaction times.

The present invention provides, therefore, a process for the preparationof refractory metal carbides by gas phase carburization of tungstenpowder and/or suitable tungsten precursor compound powders attemperatures above 850° C., which is characterised in that thecarburizing gas phase used is a CO₂/CO mixture with a CO₂ content whichis above the Boudouard equilibrium content corresponding to thecarburization temperature.

The gas phase used is preferably one which, apart from unavoidabletraces of nitrogen, argon and helium, is composed exclusively of carbondioxide and carbon monoxide.

In order to maintain the preselected CO₂—CO ratio, carbon dioxide formedis drawn off during reduction and carburization. This may be carried outby introducing carbon monoxide into the carburization reactor as afunction of the CO₂ content of the gas phase, or by flushing the reactorwith the gas phase which has the preselected CO₂—CO ratio.

The carburization temperature is preferably from 900° C. to 950° C.

The CO₂ content of the CO₂—CO mixture is preferably below 8 mole % inthe temperature range from 850° C. to 900° C. and below 4 mole % in thetemperature range above 900° C.

Carburization at carburization temperature is carried out preferablyover a period from 4 to 10 hours, particularly preferably over a periodfrom 5 to 8 hours. Within the context of the process according to theinvention, tungsten oxide powders are used preferably as the carbideprecursor. The process according to the invention is particularlypreferred if an upstream reduction of oxides or other precursorcompounds to the metal is avoided.

If other tungsten precursor compounds are used, these are decomposed tothe oxide preferably in an upstream calcining step. This has theadvantage that the carburizing gas is not contaminated by decompositionproducts and may therefore be recycled.

According to a further preferred embodiment of the invention, thetungsten carbides obtained according to the process of the inventionundergo a heat treatment at 1,150° C. to 1,800° C. after carburization.The temperature during the subsequent heat treatment is preferably1,350° C. to 1,550° C., particularly preferably up to 1,450° C. Thethermal after-treatment may be carried out, for example, in asliding-batt kiln for a period from 1 to 60 minutes, preferably 25 to 50minutes. Optionally, the heat treatment may be carried out with theaddition of carbon-containing compounds.

Sintered parts with homogeneous structures and a high degree of hardnessmay be prepared from the fine-particle carbide powders obtainableaccording to the invention, without the need for intensive work up bygrinding. Sintered hardmetal hardnesses obtained are superior to thoseof commercial grades with the same binder contents. This is also due tothe fact that the carbides obtained according to the invention exhibitlittle agglomeration and are present in a virtually uniform particlesize so that the tendency to secondary grain growth during sintering isinsignificant. Particularly sinter-stable carbide powders are obtainedby the thermal after-treatment because crystal lattice defects arelargely removed by the thermal after-treatment

The present invention also provides tungsten carbide powders with a fineprimary grain, expressed as coherence length, and high crystal quality,expressed by the lattice strain in %, lattice strain and coherencelength being determined according to B. E. Warren and B. L. Averbach,Journal of Applied Physics, 21 (1950), pp. 595–599. The tungsten carbideaccording to the invention is characterised by a relationship betweencoherence length x and lattice strain y according to the formulay<(−4.06*10⁻⁴ nm*x+0.113)%  (I).Particularly preferred tungsten carbides according to the invention havea relationship between coherence length x and lattice strain y whichfulfils the two conditions below:y<(−2.5*10⁻⁴ nm*x+0.1025)%  (IIa) andy<(−7.78*10⁻⁴ nm*x+0.1395)%  (IIb).The invention is explained in more detail below on the basis of theattached Figures.

Particularly preferred tungsten carbides are characterised by therelationship between coherence length x and lattice strain y accordingto the formulay<(1−x ²/3600 nm²)^(1/2).0.075%  (III).Tungsten carbides of this kind are obtained by heat treatment followingcarburization.

FIG. 1 shows the relationship between lattice strain and coherencelength of the tungsten carbide powders preferred according to theinvention, wherein the figures next to the measured values refer to theExamples given below, and the letters next to the measuring pointsoutside the range according to the invention refer to products availableon the market.

FIG. 2 shows an SEM photograph of a tungsten carbide powder preparedaccording to Example 2 below.

FIG. 3 shows an SEM photograph of the tungsten carbide powder preparedaccording to Example 3.

FIG. 4 shows an SEM photograph of the tungsten carbide powder preparedaccording to Example 4.

FIGS. 5 and 6 show SEM photographs of hardmetals prepared using tungstencarbide powders according to Example 1 and 3 respectively.

The invention is explained in more detail below on the basis ofExamples.

EXAMPLES Example 1

2 kg of WO₃ blue, 0.60 μm (ASTM B330), were heated to 500° C. in asinter furnace under an N₂ atmosphere. The furnace was then evacuatedand changed to CO/CO₂ process gas, the CO/CO₂ ratio being 97/3, andheated to 920° C. The carbon activity was 0.65 at reaction temperature.The CO₂ formed during the reaction was removed continuously and replacedby CO, the CO/CO₂ ratio of 97/3 being kept constant. The reaction wascompleted after 8 hours so the furnace could then be cooled under N₂ toroom temperature. About 1.5 kg of powder were obtained, which could beidentified on the basis of x-ray diffraction as pure-phase tungstencarbide. The powder was characterised by the following analyticalvalues:

C_(total) = 5.90% C_(free) < 0.02% O = 0.57% N = 0.06% FSSS = 0.47 μm(ASTM B330)

The % values given above and hereinafter refer to percentages by weight.

Example 2

2 kg of WO₃ blue were converted to tungsten carbide in a sinter furnaceas described in Example 1, operations being carried out in this case attemperatures up to 700° C. under process gas during the cooling phasebefore the furnace was allowed to cool to room temperature under N₂:

C_(total) = 5.89% C_(free) < 0.02% O = 0.41% N = 0.07% FSSS = 0.32 μm(ASTM B330)

The powder thus obtained underwent a thermal after-treatment in thesliding-batt kiln for 40 minutes at 1,400° C., the carburizingatmosphere in the furnace being sufficient for the carbon content of thetungsten carbide to approximate the theoretical. The powder obtainedtherefrom (FIG. 2) was characterised by the following analytical values:

C_(total) = 6.08% C_(free) < 0.03% O = 0.23% N = 0.05% FSSS = 0.40 μm(ASTM B330)

Example 3

Operations were carried out in a similar way to Example 2 except thatfine-particle tungstic acid (FSSS=0.40 μm, according to ASTM B330) wasused for carburization in this case. Initially, the material wascalcined in situ at 500° C. for 3 hours, then operations were continuedas in Example 2. On the basis of the SEM photograph (FIG. 3), it isevident that the powder exhibits little agglomeration.

C_(total) = 6.08% C_(free) < 0.03% O = 0.24% N = 0.05% FSSS = 0.29 μm(ASTM B330)

Example 4

Operations were carried out in a similar way to Example 3 except that,prior to the high temperature stage, 0.6% Cr₃C₂ and, in order toguarantee the theoretical carbon content, a calculated amount of carbonwas added to the carbide powder. The following powder characteristicdata were obtained:

C_(total) = 6.14% C_(free) < 0.02% O = 0.36% N = 0.05% FSSS = 0.37 μm(ASTM B330)

A fine-particle powder exhibiting little agglomeration was obtained(FIG. 4).

Example 5

Operations were carried out as in Example 3 except that the tungsticacid used had a particle size of 0.6 μm (measured by FSSS, according toASTM B330). The powder obtained was hardly agglomerated at all and waspresent in fine-particle form. The following characteristic data wereobtained:

C_(total) = 6.07% C_(free) < 0.04% O = 0.20% N = 0.05% FSSS = 0.30 μm(ASTM B330)Determination of Lattice Strain and Coherence Length

The lattice strain and coherence length of all the powder materials wasdetermined according to the method of BE Warren and BL Averbach, J.Appl. Phys. 21 (1950) 595 and plotted in a diagram. In addition, thismethod was also applied to tungsten carbide powders of different origin(powders S, N, T and D) and plotted in the diagram (FIG. 1). The valuesare summarised in the Table below:

TABLE 1 Material Lattice strain (%) Coherence length (nm) Example 1 0.0630.8 Example 2 0.07 72 Example 3 0.07 72 Example 4 0.07 56.5 Example 50.08 64.5 S 0.06 180 N 0.09 150 T 0.10 43.3 D 0.07 150Hardmetal tests:

Hardmetal tests were performed on some materials, doping being carriedout with Cr₃C₂ and VC, with a cobalt proportion of 10% in the hardmetalmixture. To this end the hardmetal mixtures were ground for 4 hours inhexane in the attritor (0.5 l; 300 g hardmetal mixture with 2,100 g ofhardmetal balls, size 3–4 mm) and sintered under vacuum for 45 minutesat 1,380° C. Some hardmetal characteristic values are summarised inTable 2.

TABLE 2 Density 4πσ₅ HV₃₀ Example g/cm³ H_(c(kA/m)) (μTm³/kg) (kg/mm²)A-porosity 1 14.48 41.4 16.6 1925 A04 ISO 4505 2 14.39 42.2 15.4 2001A04 ISO 4505 3 14.42 42.2 15.3 2001 A04 ISO 4505 5 14.44 43.0 14.5 2010A02–A04 ISO 4505 HC = magnetic coercivity, measured with a FoersterKoerzimat 1.096, in kA/m 4πσ₅ = magnetic saturation, measured with aFoerster Koerzimat 1.096, in μTm³/kg HV₃₀ = Vickers hardness, 30 kgload, in kg per mm².

The carbide powder from Example 1 which was not stabilised at hightemperature, had an increased tendency to secondary grain growth,whereas the other materials which were heat-treated, exhibited ahomogeneous structure (FIGS. 5 and 6).

1. A process for preparing tungsten carbide consisting of: (a)carburizing a material selected from the group consisting of tungstenpowder, tungsten precursor compound powder and combinations thereof, ata temperature ranging from 850° to 950°, and in the presence of acarburizing gas phase, said carburizing gas phase comprising a mixtureof CO and CO₂, said carburizing gas phase having a CO₂ content which isabove the Boudouard equilibrium content corresponding to thecarburization temperature, and wherein the carburizing step is carriedout with a carbon activity ranging from 0.4 to less than 1; and (b) heattreating the tungsten carbide formed in step (a) at a temperatureranging from 1,150° C. to 1,800° C., and at a carburizing atmospheresufficient for the carbon content of the tungsten carbide to approachthe theoretical, thereby forming tungsten carbide.
 2. The process ofclaim 1, wherein carburizing step (a) is carried out with a carbonactivity ranging from 0.4 to 0.9.
 3. The process of claim 1, whereincarburizing step (a) is conducted at a temperature of from 900° C. to950° C.
 4. The process of claim 1, wherein carburizing step (a) isconducted over a period ranging from 4 to 10 hours.
 5. The process ofclaim 1, wherein the tungsten precursor compound powder is tungstenoxide powder.
 6. A process for preparing tungsten carbide consisting of:(a) carburizing a material selected from the group consisting oftungsten powder, tungsten precursor compound powder and combinationsthereof, at a temperature ranging from 850° to 950°, and in the presenceof a carburizing gas phase, said carburizing gas phase comprising amixture of CO and CO₂, said carburizing gas phase having a CO₂ contentwhich is above the Boudouard equilibrium content corresponding to thecarburization temperature, and wherein the carburizing step is carriedout with a carbon activity ranging from 0.4 to less than 1; and (b) heattreating the tungsten carbide formed in step (a) at a temperatureranging from 1,150° C. to 1,800° C., thereby forming tungsten carbide,wherein said tungsten carbide is characterized by a relationship betweencoherence length x and lattice strain y according to Formula (I):y<(−4.06 10⁻⁴ nm⁻¹ x+0.113)%.